I have not been able to find any way to look for members with dead email addresses. When we send out mass emailings we get some feedback about the number of these cases, but that is really it. The forums are really up to the member to keep track of unfortunately. For example, when new members join, they are by default (for SPAM laws) not actively subscribed to any of the forums.
I will contact ClubExpress and see if they have had any email issues lately that would cause this.